Fall back to STAC_URL environment variable when opening a Client

[gadomski]

Related Issue(s): #47

Description: Fall back to an environment variable when opening a client. Includes a unit test.

Sidecar:

• Ensures that the from_file monkeypatch is rolled back in case of an exception.

PR Checklist:

• Code is formatted
• Tests pass
• Changes are added to the CHANGELOG

[codecov-commenter]

Codecov Report

Merging #48 (7e0aaf8) into main (77d95a2) will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #48      +/-   ##
==========================================
+ Coverage   85.09%   85.23%   +0.14%     
==========================================
  Files          13       13              
  Lines         624      630       +6     
==========================================
+ Hits          531      537       +6     
  Misses         93       93              

Impacted Files	Coverage Δ
pystac_client/client.py	98.57% <100.00%> (+0.13%)	⬆️

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